Stretching machines are instruments usable for wholly or partly restoring the planarity of sheets, plates or other similar material having a mainly planar development, after it has been subjected to rolling or molding or heat treatment.
To this end, a stretching machine typically comprises a pair of movable heads usable for gripping two opposite edges of the same material to be stretched by applying two respective and opposite traction forces. Each movable head comprises a front portion and a rear portion, which can be assembled together. Such a two-portion configuration makes the movable head easier to be moved during the production, assembly and maintenance operations of the same, for example if it is necessary to arrange the packaging and shipping of the movable head.
The front portion is provided with a mouth and clamping means active in said mouth for gripping and retaining the edge of the material to be stretched so that a traction force applied to the movable head along a longitudinal direction thereof may be transmitted to the material to be stretched. The front portion mouth normally extends according to a transversal direction orthogonal to the longitudinal direction. The longitudinal and transversal directions form a stretching plane of the stretching machine along which the sheet or plate tends to arrange itself due to the application of the traction force. The front portion of the movable head normally consists of a plurality of sheet segments parallel to and spaced apart from one another, arranged perpendicularly to the stretching plane.
In such a clamping head, the front portion and the rear portion are connected to each other so as to form a single stiff element, particularly resistant to bending, i.e. to a deformation caused by stresses orthogonal to the stretching plane. In fact, one problem of movable heads for stretching machines consists of the gap, between the front portion and the rear portion, which forms when the apparatus is operating, i.e. when the traction force is applied to the material to be stretched. Such a force deforms the sheet segments, which lose contact with the rear portion of the movable head. The gap that is created between the sheet segments and the rear portion does not affect the application of the traction force but causes dangerous backlashes in case of breakage of the sheet or if the clamping is lost.
A solution to such a problem is provided in DE 102007009139, which describes a second rear portion firmly restrained to the first front portion so as to support the sheet segments and impart bending resistance to the structure. The second rear portion consists of a pair of half shells, opposite to the stretching plane, engaged in respective recesses arranged in each sheet segment and connected to each other by means of a plurality of tie-rods orthogonal to the stretching plane.
During the operation of a stretching machine of the type described in DE102007009139, the application of the traction force by the clamping means of the clamping head causes the at least partial loss of the contact between them and the half shells of the rear portion. In the practice, two taper gaps having the vertex facing the stretching plane are formed between the front and rear portions of the stretching machine, on opposite sides with respect to the stretching plane. In order to obviate such a drawback, thus preventing the loss of contact and continuity between the front and rear portions, and ensuring the required stiffness of the system under all the operating conditions, a pair of wedges are provided, which wedges are able to be respectively pushed, by means of two respective pneumatic cylinders, into the two taper gaps, thus remaining in contact with the two inclined surfaces thereof, i.e. in contact with both the front and rear portions of the stretching machine.
The required presence of such a pair of wedges is the main drawback of the above solution. In fact, installing the wedges and the pneumatic cylinders and managing them during the stretching operations make such a stretching machine apt to be improved from the point of view of the construction simplicity and of the operating convenience.